Sarah doesn't trust people easily. [Sarah is often paranoid in Cosima's opinion, but on the other hand, Dyad really is out to get them?] If she trusts Helena, then Helena's done enough to earn it. I mean, I understand that the Inquisition's not going to just take Sarah's word for it indefinitely, but I can't imagine she's the only rifter who's ever gotten violently defensive in this situation.